Scientific work has demonstrated that different organic substances produce variations in electric potentials as they are deformed. This is the case, in particular, with muscles.
It has already been proposed that signals of muscular origin be used to control the automatic opening of a ski binding if the skier is in danger of breaking his leg. A safety device of this type forms the subject of German Pat. No. 2 121 827, and U.S. pat. Nos. 3,776,566 and 3,826,509.
However, this solution poses more problems than it solves. In fact, although any muscular activity generates a variation in electric potential, these Patents remain silent with regard to the manner of deliberately using these currents to generate a control signal for opening the fastener. Skiing is inevitably accompanied by muscular work. Now the fastener must only be opened under exceptional circumstances at the moment when the strains on the bones of the leg become abnormal.
It is consequently necessary to define a threshold from which a signal resulting from the electric potential of muscular origin is to be generated. The definition of this threshold poses extremely complex problems since the strains to which the leg is subjected and those which are exerted on the muscles are not necessarily proportional nor simultaneous.
Moreover, electric potentials of muscular origin do not permit determination of the type of stress to which the bones are subjected. The resistance of a bone is not the same if it is subjected to bending or to torsion. This means that the electric potentials of muscular origin can, in certain cases, exceed the critical threshold while the strain on the bone is perfectly tolerable whereas, in other cases, this critical threshold is not attained when the bone is subjected to a stress exceeding its critical value.
The deficiencies and dangers of the earlier systems appear immediately and seem to be inherent in the very design of this safety device since it measures electric potentials dependent on muscular activity and these potentials are considered to be characteristic of the strains exerted on the bones.
Scientific work has also demonstrated that piezo-electric effects are exhibited when rigid members made of organic materials such as wood or bone are subjected to mechanical strains. Tests have shown that the forms of signal generated by the electric potentials as a result of the strains exerted on the bones differ depending on the nature of the stress and that the amplitude of the signal is dependent both on the size of the stress and the rate of application thereof. This means that, for a given stress level, the signal measured will depend on the speed of application of the stress.